Flash Photolysis with Lamps

The history of flash photolysis is essentially that of increasing time resolution. As Lord Porter remarked in his Nobel address [14], science and technology have steadily extended the strict limits of man senses, so as to enable him, despite the 

Likewise, organic radicals were detected, giving an unprecedented support to the understanding of the nature and reactivity of such intermediates. As an example, benzyl radicals, characterized by a strong absorption around 300 nm, were generated by homolytic fission of alkylbenzenes and benzyl halides (see Fig. 6.3). Thus, flashing of toluene vapor showed some distinct absorptions, in particular one around 305 nm, which were observed also from benzyl chloride and, with very little difference, from ethylbenzene and was thus attributed to the benzyl radical (see also the following section) [19]. The improvement of lamps for a short flash (2 )is in the spectra in Fig. 6.4) allowed a more detailed examination and the detection of further bands [20]. 

Many radicals absorb in the visible and near UV region, and thus photographic detection is suitable. For those that do not, the development of appropriate lasers made it possible to observe high-resolution IR spectra within a short timescale for the study of radicals in the gas phase [23]. With the availability of relatively cheap 

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B2.5.4 FLASH PHOTOLYSIS WITH FLASH LAMPS AND LASERS... 

Fig. 7.3 Experimental setup for the nanosecond laser Flash Photolysis with a white light continuum. A Brilland-Quantel Nd YAG laser delivers the fundamental pulses (355 and 532 nm). A pulsed XBO lamp is used as white light source. The laser signal is split in order to trigger the digital storage oscilloscope (DSO) utilizing a second photodiode (PD). Two separate detection units in different geometries—photomultiplier (PMT) in front face and a PD in side face—detect the signal in the UV/vis and NIR region, respectively. The monochromator is operated by a standard PC...

The absorption spectra of the dyes were measured with a Shimadzu UV-3101 PC spectrophotometer (Japan) in a cell with a 1-cm optical path length. The fluorescence and fluorescence excitation spectra were studied with the use of a Shimadzu RF-5301 PC spectrofluorimeter. To study the triplet state of the dyes, apparatuses of flash photolysis with xenon lamp excitation (with an energy of 50 J and a pulse length at half maximum of xi/2 = 7 ps) [6] was used. To detect the triplet state of the dyes, the solutions were deoxygenated using a vacuum unit or purged with argon for experiments on the laser flash photolysis apparatus. A... 

In addition to the S( S) signal, Donovan (109) detected absorption due to vlbrationally excited (v" = 2) CO and S(3p2, Pj and Pq)> the latter in a non-Boltzmann distribution which does not appear as quickly following the flash lamp discharge as does the S( S) absorption. This delayed appearance of the S( Pj) atoms suggests that they are, at least in part, produced in reactions subsequent to the photolysis. This is confirmed by Donovan et al. (Ill) who studied the flash photolysis with added He as the buffer gas rather than with added Ar. The strong spectrum of S( Pj) observed in the presence of Ar at short delay was virtually absent in the presence of He. Thus it seems that process 16a is not an important process at the wavelengths employed by Donovan. 

One of the most important teclmiques for the study of gas-phase reactions is flash photolysis [8, ]. A reaction is initiated by absorption of an intense light pulse, originally generated from flash lamps (duration a=lp.s). Nowadays these have frequently been replaced by pulsed laser sources, with the shortest pulses of the order of a few femtoseconds [22, 64]. 

There are countless other reactions, many like these and others rather different, but the idea in every case is the same. A sudden flash of light causes an immediate photo-excitation chemical events ensue thereafter. This technique of flash photolysis was invented and applied to certain gas-phase reactions by G. Porter and R. G. W. Nor-rish, who shared with Eigen the 1967 Nobel Prize in Chemistry. High-intensity flash lamps fired by a capacitor discharge were once the method of choice for fast photochemical excitation. Lasers, which are in general much faster, have nowadays largely supplanted flash lamps. Moreover, the laser light is monochromatic so that only the desired absorption band of the parent compound will be irradiated. 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

With the invention of the laser in 1960 and the subsequent development of pulsed lasers using Q-switching (Chapter 1), monochromatic and highly-collimated light sources became available with pulse durations in the nanosecond timescale. These Q-switched pulsed lasers allow the study of photo-induced processes that occur some 103 times faster than events measured by flash lamp-based flash photolysis. 

The photolyzing light pulse is produced by a dye laser and enters the sample at about 10° to the axis of the sample beam. The observation beam originates from a 75-W xenon arc lamp. The apparatus is supplied by OLIS, Athens, Georgia USA. Reproduced with permission from C. A. Sawicki and R. J. Morris, Flash Photolysis of Hemoglobin, in Methods in Enzymology (E. Antonini, L. R. Bernardi, E. Chiancone eds.), 76, 667 (1981). 

Fig. 3.16 Time resolved ir spectra obtained by uv flash photolysis of [CpFe(CO)2l2(14) (0.6 mM) and MeCN(6mM) in cyclohexane solntion at 25°. Only 5% of 14 is destroyed by the flash so that the concentration of 16 < 14. The spectra have been reconstituted from <=70 kinetic traces recorded at intervals of 4 cm from 1750 cm to 1950 cm . The first three spectra correspond to the duration of the firing of the flash lamp and subsequent spectra are shown at intervals of 10 ps. The negative peaks in the first spectrum (subsequently omitted) are due to material destroyed by the flash. Reproduced with permission from A. J. Dixon, M. A. Healy, M. Poliakoff and J. J. Turner, J. Chem. Soc.

Excitation of molecules inherently generates new electronic species which have their own unique absorption spectra. Ordinarily, secondary absorption due to electronically excited molecules is not observed because of the extremely low steady-state concentrations formed with moderate illumination. However, there are two general methods in which the transient absorption spectra of excited molecules may be observed (a) high-intensity irradiation of a solution of the solute in a rigid matrix, and (6) flash photolysis of the solute in either solution or solid state followed by a secondary flash from an analysis lamp. 

Unfortunately, preparative experiments of Iwaoka and Kondo (35) are of no direct relation to mechanistic investigations the use of a low pressure mercury lamp provides no selectivity as far as excitation of substrate or products is concerned. However, the fact that photolysis in strong acid solution decreased the bleaching rate would indicate the absence of an anchimeric effect and the results of their investigations by flash photolysis are in agreement with the electron (Equation 2) and energy transfer (Equation 4) reactions upon direct excitation. 

The convenience of having a value of Ft(0) as large as possible has been shown in Equations 6.50 and 6.52. A common practice is to pulse a xenon lamp with a steady power of 500 W or 1 kW for periods of several milliseconds when the photomultiplier is used with only five or six dynodes. The lamp is run steadily at a power lower than 500 W or 1 kW but it reaches a much higher power, that is, several times the steady power, when pulsed. Over aperiod of several hundred microseconds, at the peak of the pulse, the intensity of the lamp remains constant. The flash photolysis experiment is timed to happen over this period when the intensity of light is constant at its maximum value. Although the lamp is being pulsed, the probe is a steady light beam for at least several hundred microseconds. 

Irradiation of 2,2-dimethyl chromene through Pyrex using a 550-W Hanovia lamp initiates a retro 4 + 2 reaction to form the extended quinone methide 4, which reacts with methanol to form a pair of methyl ethers (Scheme 6A).18 Flash photolysis of coniferyl alcohol 5 generates the quinone methide 6 (Scheme 6B) by elimination of hydroxide ion from the excited-state reaction intermediate.19 The kinetics for the thermal reactions of 6 in water were characterized,20 but not the reaction products. These were assumed to be the starting alcohol 5 from 1,8-addition of water to 6 and the benzylic alcohol from 1,6-addition of water (Scheme 6). A second quinone methide has been proposed to form as a central intermediate in the biosynthesis of several neolignans,21a and chemical synthesis of neolignans has been achieved... 

Nanosecond laser Flash Photolysis experiments were performed with 355 and 532 nm laser pulses from a Brilland-Quantel Nd YAG system (5 ns pulse width) in a front face (VIS) and side face (NIR) geometry using a pulsed 450 W XBO lamp as white light source. Similarly to the femtosecond transient absorption setup, a two beam arrangement was used. However, the pump and probe pulses were generated separately, namely the pump pulse stemming from the Nd YAG laser and the probe from the XBO lamp. A schematic representation of the setup is given below in Fig. 7.3. 0.5 cm quartz cuvettes were used for all measurements. 

Laser flash photolysis was performed with a Nd-YAG Laser from Quantel which was frequency doubled with a KDP-Crystal. Samples were excited at 532 nm and the transients formed monitored with a pulsed 250 W Xenon lamp and recorded using a monochromator and an R128 photomultiplier connected with a transient digitizer linked to a PDP11-73 minicomputer. 

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